The present invention relates to methods and apparatus for use in wireless (mobile) telecommunications systems. In particular, embodiments of the invention relate to methods and apparatus for reporting on channel conditions in wireless telecommunications systems.
Third and fourth generation mobile telecommunication systems, such as those based on the 3GPP defined UMTS and Long Term Evolution (LTE) architecture, are able to support more sophisticated services than simple voice and messaging services offered by previous generations of mobile telecommunication systems.
For example, with the improved radio interface and enhanced data rates provided by LTE systems, a user is able to enjoy high data rate applications such as mobile video streaming and mobile video conferencing that would previously only have been available via a fixed line data connection. The demand to deploy third and fourth generation networks is therefore strong and the coverage area of these networks, i.e. locations where access to the networks is possible, is expected to increase rapidly.
The anticipated widespread deployment of third and fourth generation networks has led to the parallel development of a class of devices and applications which, rather than taking advantage of the high data rates available, instead take advantage of the robust radio interface and increasing ubiquity of the coverage area. Examples include so-called machine type communication (MTC) applications, which are typified by semi-autonomous or autonomous wireless communication devices (i.e. MTC devices) communicating small amounts of data on a relatively infrequent basis. Examples include so-called smart meters which, for example, are located in a customer's house and periodically transmit information back to a central MTC server relating to the customers consumption of a utility such as gas, water, electricity and so on. Further information on characteristics of MTC-type devices can be found, for example, in the corresponding standards, such as ETSI TS 122 368 V10.530 (2011-07)/3GPP TS 22.368 version 10.5.0 Release 10) [1]. Some typical characteristics of MTC type terminal devices/MTC type data might include, for example, characteristics such as low mobility, high delay tolerance, small data transmissions, infrequent transmission and group-based features, policing and addressing.
Whilst it can be convenient for a terminal such as an MTC type terminal to take advantage of the wide coverage area provided by a third or fourth generation mobile telecommunication network there are at present disadvantages. Unlike a conventional third or fourth generation terminal device such as a smartphone, an MTC-type terminal is preferably relatively simple and inexpensive and able to operate on relatively low resources (e.g. low power consumption). The type of functions performed by the MTC-type terminal (e.g. collecting and reporting back data) do not require particularly complex processing to perform, and furthermore are typically not time-critical. However, third and fourth generation mobile telecommunication networks typically employ advanced data modulation techniques on the radio interface which can be power hungry and require more complex and expensive radio transceivers to implement. It is usually justified to include such complex transceivers in a smartphone as a smartphone will typically require a powerful processor to perform typical smartphone type functions. However, as indicated above, there is now a desire to use relatively inexpensive and less complex devices able to operate with low resource usage, to communicate using LTE type networks. To this end, so-called “virtual carriers” have been proposed.
Some characteristics of virtual carriers are discussed in more detail further below. However, in brief summary, certain classes of devices, such as MTC devices, may support communication applications that are characterised by the transmission of small amounts of data at relatively infrequent intervals and can thus be considerably less complex than conventional LTE devices. Typical LTE communications devices may include a high-performance receiver unit capable of receiving and processing data from an LTE downlink frame across the full carrier bandwidth. However, such receiver units can be overly complex for a device which only needs to transmit or to receive small amounts of data. This may therefore limit the practicality of a widespread deployment of reduced capability MTC type devices in an LTE network. It has therefore been proposed to provide reduced capability devices such as MTC devices with a simpler receiver unit which is more proportionate with the amount of data likely to be transmitted to the device. Furthermore, as explained above it is desirable to include features in a mobile communications network and/or communications devices which can conserve power consumption of the communications devices.
In conventional mobile telecommunication networks, data is typically transmitted from the network to the communications devices in a frequency carrier (first frequency range) where at least part of the data might span substantially the whole of the bandwidth of the frequency carrier. Normally a communications device cannot operate within the network unless it can receive and decode data spanning the entire frequency carrier, i.e. a maximum system bandwidth defined by a given telecommunication standard, and therefore the use of communications devices with reduced bandwidth capability transceiver units can in effect be precluded from operating with such a carrier.
However, in accordance with the previously proposed virtual carrier concepts, a subset of the communications resource elements comprising a conventional carrier (a “host carrier”) are defined as a “virtual carrier”, where the host carrier has a certain bandwidth (first frequency range) and where the virtual carrier has a reduced bandwidth (second frequency range) compared to the host carrier's bandwidth. Data for reduced capability devices is separately transmitted on the virtual carrier set of communications resource elements. Accordingly, data transmitted on the virtual carrier can be received and decoded using a reduced complexity or capability transceiver unit (i.e. one with a transceiver having a narrower operating bandwidth than would otherwise be required to operate in the network).
Devices provided with reduced complexity or capability transceiver units (hereafter referred to as “reduced capability devices”) could operate by using a part of its full capability (i.e. reduced capability set of its full capability) or they could be constructed to be less complex and less expensive than conventional LTE type devices (onwards referred to generally as legacy LTE devices). Accordingly, the deployment of such devices for MTC type applications within an LTE type network can become more attractive because the provision of the virtual carrier allows communications devices with less expensive and less complex transceiver units to be used.
Conventional LTE type networks allow for so-called link adaptation by a scheduler of a base station. Link adaptation allows a base station to modify its transmissions characteristics in a manner which takes account of channel conditions existing between the base station and a terminal device. For example, higher data rates may be used when channel conditions compared to when channel conditions are bad. A significant aspect of link adaptation is channel quality indicator (CQI) reporting. As is well established, a terminal device may measure the channel quality of a downlink communication and report it back to the base station as a CQI report. The base station may then perform link adaptation based on the CQI report.
Existing LTE standards provide for CQI reports with two types of bandwidth. One is known as wideband CQI and the other is known as sub-band CQI. For wideband CQI a single CQI value is established for a carrier's full bandwidth and reported to the base station. For sub-band CQI, the full bandwidth is in effect split into more than one sub-band, and a CQI value is established for each sub-band. The wideband CQI approach is simple and provides for compact signalling whereas the sub-band CQI approach can allow a scheduler to take account of frequency selective channel conditions (e.g. frequency-dependent fading).
The inventors have recognised that particular considerations might apply when considering channel conditions, for example through CQI measurement and reporting, in the context of virtual carriers. In principle a terminal device operating on a virtual carrier can implement CQI measurement and reporting within the virtual carrier in accordance with the same principles as used for conventional CQI measurement and reporting within a conventional carrier. However, in accordance with virtual carrier techniques there is in principle the possibility of a base station scheduler moving a virtual carrier from one frequency band to another, for example because the existing virtual carrier frequency band is subject to poor channel conditions. However, there is currently no mechanism for providing a base station scheduler with information to allow the base station scheduler to determine whether or not it would be appropriate to move a virtual carrier from one frequency to another.
There is therefore a desire to provide for improved schemes for reporting on channel conditions in wireless telecommunications systems.